B&W 800d's or MAGICO V3's?

They are both dynamic designs at roughly the same price/quality level. No question the 800d's are better for HT than the V-3 and are as good (only different) for audio. There are lots of sideway moves at your system level. You better love the next pair of speakers before you move to them.

Why are you considering this specific change?

Magico speakers are really something special. They don't sound like conventional box speakers, more like dipole ESL or planars, with the dynamics of a good box speaker. They take advantge of both worlds. Very good design.
I wish I could afford one pair . For now I enjoy electrostatics with tube amplification and this combo is also magical with vinyl. Have a listen to ML Summits or Magnepan 20, Magic....ohhhh !!!!

Your ears.

Your money.

Your decision.

"Magico speakers are really something special. They don't sound like conventional box speakers, more like dipole ESL or planars, with the dynamics of a good box speaker. They take advantge of both worlds."

Never heard Magicos but from this description, they are in a different camp than B&Ws for sure. Sounds like a personal preference choice, not a matter of which is better.